As conventional semiconductor memory, such as Flash memory and dynamic random access memory (DRAM), reach their scaling limits, research has focused on commercially viable low power, low operation voltage, high speed and high-density non-volatile memory devices. One example of such a non-volatile memory device is a variable resistance memory device including a programmable resistive memory material formed from a material exhibiting a very large negative magnetoresistance, often referred to as a so-called “colossal magnetoresistance” (CMR) material. The CMR material may be connected to a current controlling device, such as a diode, a field effect transistor (FET), or a bipolar junction transistor (BJT).
The resistance of the CMR material remains constant until a high electric field induces current flow through the CMR material, resulting in a change in the CMR resistance. During a programming process, the resistivity of the memory resistor at the high field region near the electrode changes first. Experimental data shows that the resistivity of the material at the cathode is increased while that at the anode is decreased. During an erase process, the pulse polarity is reversed. That is, the designation of cathode and anode are reversed. Then, the resistivity of the material near the cathode is decreased, and the resistivity near the anode is increased.
One example of a CMR material is a manganese oxide of the general formula R1-xMxMnO3, wherein R is a rare earth element, M is a metal (e.g., calcium, strontium or barium), and x is a number from about 0.1 to about 0.9. The CMR material is often referred to as “CMR manganites.” CMR manganites exhibit reversible resistive switching properties, which may be used for low power, low operation voltage, high speed and high-density memory applications.
PrCaMnO (PCMO) is a CMR manganite that is currently being explored due to its potential for use in variable resistance memory devices. Amorphous PCMO may be deposited using a variety of methods, such as physical vapor deposition (PVD), metal-organic chemical vapor deposition (MOCVD), and spin-coating. However, the resistive switching characteristics of PCMO have been shown to improve as the PCMO reaches a crystalline phase. To convert amorphous PCMO to the crystalline phase such that the PCMO exhibits properties useful in variable resistance memory devices, the amorphous PCMO may be exposed to temperatures of greater than about 400° C. For example, after depositing the amorphous PCMO using a conventional CVD process, an annealing process is performed to convert the amorphous PCMO to crystalline PCMO by exposing the amorphous PCMO to a temperature of about 525° C.
Alternatively, an MOCVD process may be performed at increased temperatures (i.e., about 600° C.) to form crystalline PCMO. However, PCMO materials formed at temperatures greater than 550° C. may exhibit decreased resistive switching characteristics. While not wishing to be bound by any particular theory, it is believed that decomposition of the MOCVD reactants may lead to uncontrolled growth of the PCMO. Precursors for depositing PCMO, such as bis(2,2,6,6-tetramethyl-3,5-heptanedionato)calcium (Ca(tmhd)2), Pr(tmhd)3, and Mn(tmhd)2, have been explored for use in CVD processes. Due to their low reactivity, the precursors are deposited at increased temperatures and are codeposited with oxygen. Accordingly, depositing materials by ALD using such precursors may be difficult to control or altogether unsuccessful.
Exposing semiconductor memory of memory device structures to increased temperatures during fabrication may cause degradation of heat-sensitive components, such as metal wiring and interconnects. Thus, it is desired to conduct semiconductor memory fabrication acts at relatively low temperatures (e.g., less than about 450° C.) to prevent such degradation.